net/http/httputil: ReverseProxy calls WriteHeader multiple times on a single response

[aakog]

What version of Go are you using (go version)?

$ go version
go version go1.23.2 darwin/arm64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/aaron/Library/Caches/go-build'
GOENV='/Users/aaron/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/aaron/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/aaron/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.23.2/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.23.2/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.2'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/aaron/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/aaron/kognitos/brain/aiproxy/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/gv/b9j8dl_j2gz5d_6n94m3qmj80000gn/T/go-build520885361=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOROOT/bin/go version: go version go1.23.2 darwin/arm64
GOROOT/bin/go tool compile -V: compile version go1.23.2
uname -v: Darwin Kernel Version 24.0.0: Mon Aug 12 20:52:12 PDT 2024; root:xnu-11215.1.10~2/RELEASE_ARM64_T6020
ProductName:		macOS
ProductVersion:		15.0
BuildVersion:		24A335
lldb --version: lldb-1600.0.36.3
Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)

What did you do?

https://go.dev/play/p/P_8ZOOmh4UV

What did you expect to see?

response.WriteHeader should be called exactly once with 502 as the status code.

What did you see instead?

ReverseProxy calls response.WriteHeader multiple times on one response.

http: superfluous response.WriteHeader call from net/http/httputil.(*ReverseProxy).defaultErrorHandler (reverseproxy.go:308)

[gabyhelp]

Related Issues and Documentation

• net/http/httputil: ReverseProxy copyHeader() appends to pre-existing headers instead of overwriting them from target service's response. #66995 (closed)
• net/http: TimeoutHandler doesn't log superfluous WriteHeader calls #30803 (closed)
• net/http/httputil: ReverseProxy does not close response body if ModifyResponse returns an error #22658 (closed)
• net/http: Response.Write adds redundant "Connection: Close" headers #19499 (closed)
• net/http: ResponseController may cause infinite loop #66350 (closed)
• net/http/httputil: ReverseProxy doesn't handle connection/stream reset properly #34413
• x/net/webdav: `WriteHeader` may be called twice when webdav processes methods such as `PROPFIND` #66085
• net/http/httputil: ReverseProxy fails to proxy h2c #33452 (closed)
• net/http/httputil: ReverseProxy does not flush header writes #31126 (closed)
• net/http/httputil: httputil.ReverseProxy does not call ModifyResponse on failed requests #21255 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[aakog]

Some more context here: I'm using httputil.ReverseProxy and I want to record a metric once per request which tracks the result of the request. ReverseProxy.ErrorHandler seems like a reasonable place to add such a metric for the case when the request fails. However, it's not clear from the docs if ErrorHandler could be called more than once for a single request.

Looking at the code, it turns out that ErrorHandler will be called multiple times in some cases. One such case is here https://go.dev/play/p/P_8ZOOmh4UV

On the other hand, ReverseProxy.defaultErrorHandler should not be called more than once per request. It calls http.ResponseWriter.WriteHeader which requires "at most one 2xx-5xx header".

Ideally the ReverseProxy interface would be clarified so that it is simple to track how many requests failed.

[gopherbot]

Change https://go.dev/cl/627635 mentions this issue: net/http/httputil: return after handling error